Method for non-evasively determining an endothelial function and a device for carrying out said method

ABSTRACT

The invention relates to medicine. The inventive method for non-evasively determining an endothelial function consists in reducing a transmural pressure in a limb, recording the amplitude of plethysmographic signals at different pressures, determining the pressure of a maximum plethysmographic signal amplitude, reducing the pressure to a value corresponding to the specified percentage of the maximum amplitude, carrying out an occlusion sample in the course of which a pressure higher by at least 50 mm Hg than the systolic pressure of a tested patient is produced in a cuff arranged proximally to the located limb area, wherein the occlusion is carried out during at least 5 minutes. The inventive device for carrying out said method comprises a sensory unit, a pressure producing unit, an electronic unit constructed in such a way that it makes it possible to measure the cuff pressure corresponding to the maximum plethysmographic signal amplitude and a unit for controlling the pressure producing unit in such a way that the cuff pressure corresponding to the amplitude of a plethysmographic signal, which represents a specified percentage of the maximum amplitude, is attained. The sensory unit is connected to the electronic unit to the output of which the pressure producing unit is connected.

BACKGROUND OF THE INVENTION

Area of application: The invention applies to medicine, particularly tofunctional diagnostics, and allows early detection of cardiovasculardiseases as well as the monitoring of the effectiveness of treatmentreceived by the patient. The invention assesses the state of endothelialfunction and, based on this assessment, addresses the need of earlydiagnosis of cardiovascular disease. This invention can be used forwidespread testing of the population.

Prior Art: As of late, the need for early cardiovascular diseasedetection has become more and more relevant. For this end a widespectrum of means and methods described in patent and scientificliterature is used. For example, the U.S. Pat. No. 5,343,867 describes amethod and apparatus for early diagnosis of atherosclerosis usingimpedance plethysmography to detect an irregular pulse wave in thearteries of the lower extremities. It is shown that blood flowparameters depend on the outside pressure applied to the artery underinvestigation. The maximum amplitude of the plethysmogram is in largedetermined by the size of the transmural pressure, which is defined asthe difference between internal arterial pressure and the pressureapplied externally by the cuff. Maximum amplitude is reached when thetransmural pressure drops to zero.

From the perspective of the structure and physiology of blood vessels,this can be imagined to occur in the following manner: Pressure from thecuff transfers to the exterior of the artery and counterbalances thepressure of the interior of the artery wall. With this, the complianceof the arterial wall increases dramatically and the passing pulse wavestretches the artery to a large diameter, i.e. the increase of thearterial diameter at the same pulse pressure becomes significant. Thisphenomenon can be observed on an oscillometric curve, recorded duringthe taking of arterial pressure. The peak of the oscillometric curvecorresponds to the moment when the pressure inside the cuff equals themean arterial pressure.

U.S. Pat. No. 6,322,515 describes a method and apparatus for determininga set of cardiovascular parameters which are used for the evaluation ofthe state of endothelial function. Photodiodes and photoreceptors areused as pulse wave sensors, and an analysis of photo-plethysmographicwaveforms is made. The measurements for these waveforms are taken at theobserved artery before and after the test with reactive hyperemia.During the recording of these waveforms, the cuff, where the pressure ismaintained at 70 mmHg, is placed on the digit, over the optical sensor.

U.S. Pat. No. 6,939,304 reveals a method and apparatus for non-invasiveassessment of endothelial function using the photoplethysmography (PPG)sensor.

U.S. Pat. No. 6,908,436 reveals a method for the assessment ofendothelial function by measuring the spreading of the pulse wave. Forthis a two-channel plethysmograph is used, sensors are mounted on one ofthe phalanges of the finger, and an occlusion is created with the helpof the cuff placed on the shoulder. Changes in the state of the arterialwall are assessed based on the duration of delay in the spreading of thepulse wave. If the duration of the delay is 20 ms or longer, the delayis considered to confirm normal endothelial function. The duration ofthe delay is established by comparing it with the PPG waveform which ismeasured on the arm not influenced by the occlusion test. However, thismethod falls short when it comes to determining the delay ofdisplacement in the area of the minimum directly before systolic growth,i.e. in the area which is to a significant extent variable.

The most analogous method and apparatus to the one described below isthe method and apparatus for non-invasive evaluation of the physicalstate of the patient described in patent #2,220,653 of the RussianFederation. The method consists of the following: First a control of theperipheral arterial tone is established by distributing the pulse of thecuff across several sensors and raising the pressure in the cuff to 75mmHg. Then arterial pressure is measured by raising the pressure in thecuff above the systolic pressure and keeping it there for five minutes.Pulse wave measurements are further taken on both arms by the PPGmethod. Finally the PPG waveform is analyzed with regard to itsamplitude by comparing its values before and after the occlusion anddetermining the increase of the measured output. This apparatus includesa sensor for measuring pressure with the cuff, a heating element forheating the surface of the observed region of the body, and a processorfor processing the measured output.

However, this method and this apparatus are not able to provide reliableresults due to the low precision of measurements and their dependency onthe fluctuating blood pressure of the patient.

BRIEF SUMMARY OF THE INVENTION

The endothelial dysfunction occurs in the presence of various riskfactors for cardiovascular disease (CVD), such as high cholesterollevels, arterial hypertension, smoking, age and others. It isestablished that endothelial cells is the organ-target for thepathogenic realization of factors contributing to the risk of CVDdevelopment. The assessment of endothelial function acts as the“barometer” which allows early diagnosis of CVD. Such a diagnosticapproach will allow a departure from the current approach were a set ofbiochemical tests must be administered to the patient (determining thelevels of cholesterol, high and low-density lipoproteins, and others )in order to detect the presence of risk factors. During the early stagesof CVD it is economically sound to screen the population using internalindicators for the risk of disease development. One such indicator isthe state of endothelial function. The assessment of this state is alsoextremely relevant for the quality assessment of received therapies.

The goal of this invention is to create a physiologically-basednon-invasive method and apparatus for reliably determining the state ofendothelial function in the patient. This method and apparatus willoffer an individualized approach based on the particular conditions ofthe patient. The method and apparatus will be based on a system ofconversion, amplification, and recording of the PPG output during theonset of the optimal magnitude of the established pressure or pressurelocally applied to the observed artery before and after the occlusiontest.

The technical result, which is achieved with the use of theaforementioned device and apparatus, is centered on reliable assessmentof endothelial function regardless of the arterial pressure of thepatient.

In terms of method, the technical result is achieved through a series ofsteps. First, the transmural pressure in the artery is lowered. Then theamplitude of plethysmographic output is measured at various pressures.Once the pressure at which the amplitude of the PG signal is maximal isestablished, the pressure is lowered to a predetermined percentage ofthe maximal amplitude. Finally, an occlusion test is performed for atleast five minutes. During this test a pressure is created in the cuffwhich is placed on the observed region of the extremity. This pressuremust exceed the measured systolic pressure by at least 50 mmHg.

The technical result is amplified because the transmural pressure islowered when the pressure-generating cuff is placed on a particularregion of the extremity.

The pressure on the extremity is raised gradually every five to tenseconds by increments of 5 mmHg. At each step the amplitude of the PGoutput is measured and recorded.

A mechanical pressure is locally applied to the extremity in order tolower the transmural pressure in the observed artery.

In order to lower the transmural pressure in the observed artery, thehydrostatic pressure is lessened by raising the extremity a specifiedheight above the level of the heart.

The intensity of the transmural pressure is established when theamplitude of the PG output is 50% that of the maximal PG output. Thispressure is created in the cuff which is placed in close proximity tothe observed artery. Then super-systolic pressure is created and theplethysmographic output is recorded.

After at least five minutes of exposure to the occlusion cuff placednear the observed artery, the pressure in the cuff is lowered to zero.The changes in the PG output reading are recorded simultaneously by thereference channel and the monitoring channel for at least five minutes.

After the occlusion test, the recorded plethysmographic signal isanalyzed using both the amplitudal and timely analyses based on the datacollected through the reference and monitoring channels.

During the amplitudal analysis the heights of the amplitude of theoutput from the reference channel are compared to those collected fromthe monitoring channel. Also, the growth rate of the amplitude in themonitoring channel is analyzed. Finally, the amplitude of the outputrecorded during various transmural pressures is compared to the maximalamplitude of the output recorded after the running of the occlusiontest.

During the timely analysis, plethysmogrphic waveforms collected throughthe reference and monitoring channels are compared. The output is thennormalized and the delay time or phase change is determined.

The technical result in the device is achieved by the three parts, orblocks, of the device. The first of these is a double-channeled sensorblock which detects pulse wave signals from peripheral arteries. Thesecond is a pressure block which creates gradually increasing pressurein the blood pressure cuff. The final block is an electronic block whichmeasures the pressure created in the cuff. This pressure corresponds tothe maximal amplitude of the PG signal. Along with measuring pressure inthe blood pressure cuff, the electronic block also operates the pressureblock, which creates pressure in the cuff equal to an assignedpercentage of the increase in maximal amplitude. The sensor block isconnected to the electronic block, which is in turn hooked up through anoutlet to the pressure block.

The technical result is amplified because the pressure block is able tocreate pressure which is increased gradually by increments of 5 mmHg andtime increments of five to ten seconds, in the cuff.

The sensor block in each channel includes an infrared diode and aphotoreceptor. Both of these devices are situated so that they are ableto read the light signal passing through the observed field.

The infrared diode and the light receptor are also able to sense andrecord the diffused light signal reflected off the observed field.

The sensor block also includes impedancemetric electrodes, or Hallsensors, or an elastic tube filled with electro-conductive material.

The light receptor is connected to a filter which filters pulse wavecomponent out of the general signal.

The device also includes orthogonally-placed polarized filters whichprotect the photoreceptor from extraneous exposure to light.

The sensor block finally includes a means for maintaining a settemperature of the observed region of the body.

The device has either a liquid-crystal screen for displaying the resultsof endothelial tissue assessment, or an interface system connected tothe electronic block for transferring the collected data to a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

The essence of the invention and its ability to provide certain resultswill be easier understood through an example of usage. In this examplethere will be references made to the drawings below.

FIG. 1 illustrates the dynamic between measurements of total blood flowand the diameter of the shoulder artery during the occlusion test.

FIG. 2 shows a diagram of the formation of a PPG waveform.

FIG. 3 shows a PPG curve while

FIG. 4 shows a family of such waveforms recorded at various magnitudesof transmural pressure in patients of the control group. Finally,

FIG. 5 illustrates the effect of increased hydrostatic pressure on theamplitude of the PPG signal, and

FIG. 6 shows the major block diagram of the device.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus (FIG. 6) is used for assessing the function of endothelialtissue. The device includes a double-channeled sensor block (forsimplicity the double channel is shown as single in the figures ). Eachof the channels has an infrared light diode (2) which is connected tothe output of the control (9). It also includes a photoreceptorconnected to the input of the electric voltage converter (4). The sensorblock records pulse waveforms from peripheral arteries. The pressureblock includes a compressor (11), which creates pressure in the cuff(1). This compressor is controlled by the control (9). The pressureblock also has a pressure sensor (12) for recording the patient's bloodpressure. This pressure sensor (12) is attached to the output of thecontrol (9). Apart from this, the pressure block can be used as a meansfor applying mechanical pressure on a particular section of anextremity. The electronic block has a programmable control (9),programmed for operating the compressor (11) and turning on the lightdiode (2), as well as for processing data coming in from the lightreceptor chain. The input to the control (9) is connected to the outputof the pressure sensor (12), which in turn is connected to the converter(4) of the digital analog converter (8) through a sequence of amplifier(5), filter (6), and scalar amplifier (7). On top of that, the control(9) is connected through a set of input-output linkages to a USBinterface for connecting to an external computer (not shown ). Thephotoreceptor (3) signal is converted as it moves through a chain ofmechanisms which include the converter (4), amplifiers (5 and 7), andfilter (6) which can filter the pulse signal out from the overall noise.The device also includes orthogonally-stationed polarized filters (notshown ) for filtering out extraneous light which could overexpose thephotoreceptor (3). Finally, the device has, in its sensor block, a meansof maintaining an assigned temperature of the observed region of thebody, as well as a liquid crystal screen connected to the control (9)which displays the results of endothelial function assessment. The cuff(1) is placed over the sensors (2 and 3). In the case where mechanicaleffort is needed, a drive (for example, one that turns on the panelwhich acts on the sensors ) that is connected to the means of creating amechanical pressure is placed over the two sensors instead of the cuff.

The electronic block defines the pressure in the cuff (1) whichcorresponds to the maximal amplitude of the PG signal. This block alsocontrols the pressure block which generates pressure in the cuff (1)equal to a predetermined percentage (50%) of the maximal growth of thepressure magnitude. The sensor block can be created with two differentvariations. The first variation has the infrared light diode (2) and thephotoreceptor (3) stationed on opposite sides of the observed region ofthe extremity in order to be able to record light signals passingthrough the observed field. The second variation has the infrared diode(2) and the photoreceptor (3) stationed on the same side of the observedvessel.

Also, the sensor block can be made with impedancemetric electrodes, Hallsensors, or an elastic tube filled with electro-conductive material.

The assessment of endothelial function is founded on the PG outputreadings obtained by the sensor block which is set up on the upperextremity of the observed patient. The signal coming from the cuff iselectrically converted as the pressure increases linearly in the cuff(1) until maximal amplitude of the signal is reached. After this themagnitude of the pressure in the cuff or the locally-applied pressure isfixed and the occlusion test is run. During this procedure the sensorblock is set up on the internal side of the cuff (1) or is placed at theend of the device which applies pressure in the area where the arteryprotrudes on the surface of the skin. A reverse connection based on theamplitude of the PG output is used for automatically establishing theaforementioned pressure. During this procedure the PG signal is sentfrom the digital analog converter (8) through control (9) to thecompressor (11) located in the pressure block.

The occlusion test is done using the cuff which is in close proximity(shoulder, forearm, wrist) to the observed artery (shoulder, radial, ordigital artery ) At the same time the signal received from the otherextremity, where the occlusion test is not done, is used as a reference.

The method for assessing the state of endothelial activity of theobserved patient has two major stages. During the first stage, a set ofplethysmographic waveforms is recorded as the cuff (1) generates variouspressures (or various pressures are applied to the observed artery ).The second stage comprises the occlusion itself. The first stageprovides information on the elastic qualities of the arterial channel.This information is then used to decide between applying cuff pressureor external pressure during the occlusion test. The measurements of theamplitude of the PG signal taken during the effects of the appliedpressure describe the tone quality of smooth muscle in the artery aswell as its elastic components (elastin and collagen ). Locally-appliedpressure us accompanied by changes in transmural pressure, whosemagnitude is determined by the difference between arterial pressure andthe externally applied pressure. When transmural pressure is weakened,the muscle tone of smooth muscle tissue lowers as well. This lowering isaccompanied by the widening of arterial channels. Conversely, whentransmural pressure is raised, the arteries become narrower. In thislies the myogenic regulation of blood flow, aimed at maintaining optimalpressure in the microcirculatory system. This is the reason why duringchanges in the magistral vessel from 150 mmHg to 50 mmHg, the capillarypressure remains practically unchanged.

Changes in the muscle tone of smooth muscle tissue are expressed notonly in the narrowing or dilation of arteries, but also in the increasedstiffness or compliance of artery walls. Which the lowering oftransmural pressure, the smooth muscle apparatus of blood vessel wallsrelaxes to a certain extent. This relaxation can be seen on the PPGwaveform in the form of an increased signal amplitude. Maximal amplitudeis reached when the transmural pressure drops to zero. Schematicallythis is shown in FIG. 4, where the S-shaped curve reveals that themaximal increase of volume is defined at the point where the transmuralpressure is close to zero. When pulse waves are applied evenly tovarious points on the deformation curve, the maximal plethysmographicsignal is observed in the area where transmural pressure is nearingzero. When changing transmural pressure, the amplitude of the waveformcan increase by over 100% in patients of the control group. The patientsin the control group correspond in age and diastolic blood pressure tothe patient group showing symptoms of ischemic disease (FIG. 4). Whereasin the patient group diagnosed with ischemic heart disease, theamplitudal growth does not exceed 10-20%.

This dynamic of the changes in the amplitude of the signal duringdifferent transmural pressures can be linked only to particulars ofvisco-elastic qualities of the arterial channel in healthy individualsas well as in individuals suffering from arteriosclerosis in variouslocations. The smooth muscle tension of the artery can be considered thepredominantly viscous component, while the strands of elastin andcollagen clearly serve as the elastic components in the structure ofvessel walls. By lowering the smooth muscle tension as the reading ofthe transmural pressure approaches zero, we lessen the input of theviscous component of smooth muscle to the deformation curve. Thisdetailed technique allows a more thorough analysis of the deformationcurve of elastic components of artery vessel walls. Also this techniqueis more advantageous for recording phenomena of reactive hyperemia whichoccurs after the occlusion test.

The magnitude of diameter increase in the observed artery is believed tobe linked to the functioning of endothelial cells. The increase of thepressure during the occlusion test results in an increase of nitricoxide (NO) production by endothelial cells. This phenomenon is called“flow-induced dilation”. If the normal function of endothelial cells isdeteriorated, their ability to produce nitric oxide, along with othervaso-active compounds, is diminished. This in turn blocks thevasodilation of vessels. In this situation a full-fledged reactivehyperemia does not occur. At this time this phenomenon is used to revealthe disruption of normal endothelial function, i.e. to revealendothelial dysfunction. The flow-induced dilation of the vessel occursas a result of the following order of events: occlusion, blood flowincrease, change in pressure acting on endothelial cells, nitric oxideproduction (as well as adaptation to the increased blood flow ), andfinally nitric oxide acting on the smooth muscle.

Maximal blood flow volume is reached 1-2 seconds after the removal ofthe occlusion. It is important to note that during simultaneousmonitoring of both blood flow volume and artery dilation, the blood flowincreases first and only then does the diameter of the vessel change(FIG. 1). The maximal blood flow speed is reached quickly (within a fewseconds ), and the increase in artery diameter follows, reaching itsmaximum in one minute. After that the vessel returns to its originalsize within 2-3 minutes. Based on the particular state of the elasticmodule of artery walls in patients suffering from arterial hypertension,it is possible to make an assumption on the potential involvement ofexcessively stiff arteries in the expression of the endothelial cellresponse to the occlusion test. One cannot exclude the possibility thatwith the endothelial cells producing equal amounts of nitric oxide< theresponse of smooth muscle cells in the artery will be determined by theinitial state of the elastic module in arterial walls. In order tonormalize the response of the smooth muscle apparatus in arterial walls,it is desirable to have an identical, or if not identical at leastsimilar, level of artery stiffness in various patients. One way toprovide a uniform initial state of arterial walls is to select themagnitude of transmural pressure at which the arteries highest level ofcompliance is reached.

The assessment of the occlusion test results, in terms of reactivehyperemia, can be done not only in the shoulder artery, but in smallervessels as well.

In order to measure flow-dependent dilation, the optical method wasused. This method is based on the increase in the observed artery. Theincoming pulse wave stretches the artery walls, thereby increasing thediameter of the vessel. Since during the PPG the optical sensor recordsthe increase in blood flow, (rather than changes in the vessel diameter), which is equal to the square of the radius, the measurement can bemade with a high level of precision. FIG. 2 shows the method forobtaining the PPG signal. The photo diode records the light stream whichpasses through the observed region of the finger. As the artery widenswith each pulse wave, the volume of blood passing through it increases.Since hemoglobin in the blood absorbs significant amounts of infraredradiation, this reaction leads to an increase in optical density. Thepulse wave increases the diameter of the blood vessel as it passesthrough it. This phenomenon is the main component of the increase ofblood volume in the observed region.

FIG. 3 shows a PPG curve. There are two peaks on the curve—the firstrepresents the contraction of the heart, the second shows the affect ofthis contraction on the pulse wave. The given curve was obtained withthe optical sensor placed on the last of the phalanges of the finger.

Before taking the initial measurements, the compressor (11), followingthe signal of the control (9), generates a pressure in the cuff (1). Thepressure increases gradually, with each step of 5 mmHg lasting 5-10seconds. As the pressure increases, the transmural pressure drops,reaching zero when the pressure in the cuff equals that of the observedartery. At each step the PPG signal coming from the photoreceptor (3) isrecorded. The signal sent from the converter (4) is increased in theamplifier (5) and filtered in the filter (6) where static having theindustrial frequency of 50 Hz, along with its harmonics, is screenedout. The most significant signal amplification is created by the scalar(instrumental) amplifier. The amplified voltage is sent to the digitalanalog amplifier (8) and then through the USB interface (10) into thecomputer. The control (9) determines the pressure at which the signalreaches maximum amplitude. In order to better distinguish between thesignal and background static, the measurements are taken synchronously.

The procedure for assessing endothelial activity can be divided into twoparts:

1. The transmural pressure is lowered by external pressure applied to aregion of the finger (either by the cuff using air, an elastic band, ora mechanical compression ), or by changes in the hydrostatic pressure asthe extremity is raised to a particular height. The latter procedure canfully replace the pressure applied from the exterior to the vessel wall.In the simplified version of endothelial state assessment, one caneliminate the complex automated system and simply raise and lower thearm. As this is done, one can measure the average blood pressure basedon the maximal amplitude of the plethysmographic signal, locate thelinear section of the compliancy curve (50% of the maximal growth ), andthen carry out the occlusion test. The only shortcoming in this approachis the necessity to raise the arm and especially having to run theocclusion test with the arm still in that upraised position.

As the transmural pressure is lowered, the pulse component of the PPGgrows. This growth corresponds to the increase in the compliancy of theobserved artery. When the gradually-increasing pressure is applied tothe finger, one can see the expression of the auto-regulated reaction,and, accordingly, select the optimal conditions (based on the magnitudeof the transmural pressure ) for the occlusion test (by choosing thesteepest part of the curve describing the compliancy of the artery ).

2. The artery is closed off by applying super-systolic pressure (of 30mmHg) for five minutes. After rapidly decreasing the pressure in thecuff set up on the observed artery, the dynamic of the PPG waveform isrecorded (with the amplitudal and timely analyses in mind). The changesin the PG signal are recorded simultaneously along the reference andmonitoring channels for at least three minutes. During the amplitudalanalysis, the magnitude of the amplitudal signal is compared between thereference and monitoring channels, the rate of amplitudal signalincrease in the monitoring channel is analyzed, and the relationshipbetween amplitudal signals is established. Also, the maximal amplitudemeasured during various transmural pressures is compared to the maximalmagnitude of the signal recorded after the occlusion test. During thetimely analysis the plethysmographic waveforms measured through thereference and monitoring channels, are compared, the signal isnormalized, and finally the time of delay or the phase shift isestablished.

The amplitude of the PPG signal was maximal when the transmural pressurewas at zero (that is, the pressure applied to the vessel externally wasequal to the average arterial pressure). The calculation was made asfollows: diastolic pressure plus ⅓ of the pulse pressure. This arterialresponse to the externally applied pressure is not dependent on thestate of endothelial tissue. The method of choosing the pressure to beapplied externally to the artery allows one to test the dynamic of thePPG waveform at the moment of reactive hyperemia in the most optimalarea of the artery's compliancy. Also this method has its own intrinsicdiagnostic value: Information concerning the rheological characteristicsof the artery can be derived from the measurements of a family of PPGcurves that are taken at various transmural pressures. This informationhelps separate the changes linked to the auto-regulative effect of thesmooth muscle apparatus in artery walls when it comes to increasing theartery diameter, from the elastic attributes of the artery itself. Thediameter increase in the artery, caused by the increased volume of bloodfound in the observed region, leads to the growth of the constantcomponent of the curve. The component of the curve which represents thepulse also reflects the increase of blood flow volume into the systole.The amplitude of the PPG is determined by the level of compliancy of thearterial wall as pulse wave pressure passes through it. The open spaceinside the artery itself does not affect the amplitude of the PPGsignal. Despite some observed correlation between the one and the other,a complete correlation between the diameter of the vessel and its levelof compliancy during the measuring of the transmural pressure is notobserved.

At low transmural pressure the artery wall becomes less stiff than it isat normal physiological levels of arterial blood pressure.

The optimization of the transmural pressure test significantly increasesits sensitivity and thus allows the detection of pathologies at theearliest stages of endothelial tissue dysfunction. The high sensitivityof the test will effectively assess the success of pharmacologicaltherapy administered to the patient and directed toward the correctionof endothelial function.

When pressure in the cuff was raised to 100 mmHg, the output was alsoconstantly increasing, reaching its amplitudal maximum at 100 mmHg.Further increase in cuff pressure lead to a decrease in the amplitude ofthe PPG output. If the pressure was lowered to 75 mmHg, the amplitude ofthe PPG output dropped by 50%. Likewise, pressure in the cuff alteredthe shape of the PPG waveform (refer to FIG. 3).

The change in the shape of the PPG signal consisted of a delayed yetmore rapid and drastic systolic growth. These changes reflect the effectof the cuff on the pressure of the blood passing through the observedvessel. This occurs because the magnitude of pressure cause by the cuffis subtracted from the pulse wave pressure in the vessel.

The raising of the aim above the “point of equal blood pressure” (abovethe level of the heart ) makes the use of externally-applied pressurecaused by the cuff unnecessary. When the arm is thus raised above the“point of equal blood pressure” to the point of pointing straight up,the position of the arm increases the amplitude of the PPG output. Whenthe arm is lowered to its initial level the PPG output also decreases toits initial level.

The gravitational force is an important factor affecting the transmuralpressure. For instance, the transmural pressure in the digital artery ofthe raised arm is less than the transmural pressure in that same arterywhen at the level of the heart. The extent to which the transmuralpressure changes depends on the density of the blood, the gravitationalforce acting on the blood, and the distance that is it from the “pointof equal blood pressure”.

Ptrh=Ptrho−pgh

Where Ptrh stands for the transmural pressure in the digital artery ofthe raised hand, Ptrho—the transmural pressure in the digital artery atthe level of the heart, p—the density of blood (1.03 g/cm),g—gravitational force (980 cm/sec.), h—distance from the point of equalblood pressure to the digital artery in the upraised arm (90 cm.). Withthe given distance from the “point of equal blood pressure” the bloodpressure of the person standing with the upraised arm is 66 mmHg lowerthan the average blood pressure in the digital artery, measured at thelevel of the heart.

In this manner the transmural pressure can be lowered either byincreasing the pressure from exterior or by lowering the pressure in thevessel itself. It is relatively simple to lower the pressure in thedigital artery: one has to raise the hand above the level of the heart.By gradually raising the arm we lower the transmural pressure in thedigital artery. At the same time the amplitude of the PPG outputincreases significantly. The average pressure in the upraised arm candrop down to 30 mmHg, whereas when the hand is on the level of theheart, the pressure reads at 90 mmHg. Transmural pressure in the regionof the knee can be four times as large as the transmural pressure in thearteries of the raised arm. The effect of the hydrostatic pressure onthe magnitude of the transmural pressure can be used when assessing thevisco-elastic qualities of the artery wall.

The aforementioned inventions have the following advantages:

1. The pressure for the occlusion test is determined individually foreach patient.

2. Information about the visco-elastic attributes of the arterialchannel is provided (depending on the amplitude of the PG output derivedfrom blood pressure increase ).

3. Provide a clearer relationship between output and noise/static.

4. The occlusion test is done in a more a more optimal region of thecompliancy of the vessel.

5. The inventions provide information about the rheologicalcharacteristics of the artery by taking and recording sets of PPGwaveforms during various transmural pressures.

6. The inventions increase the sensitivity of the test, therebyincreasing the reliability of endothelial tissue function assessment.

7. Allow detection of a pathology at the earliest stages of disruptedendothelial function.

8. Allow to reliably assess the effectiveness of prescribedpharmaceutical therapies.

1. Method for noninvasive assessment of endothelial function, comprisingthe lowering of the transmural pressure in the extremity, themeasurement of the amplitude of the plethysmographic signal at variouspressures, the detection of the pressure at which the amplitude of theplethysmographic signal is maximal, the lowering of the pressure to apredetermined percentage of the maximal amplitude, the running of theocclusion test, wherein pressure exceeding the measured systolicpressure is generated in the cuff positioned in the proximity of theobserved region of the extremity, wherein a pressure is generatedexceeding the measured systolic pressure by at least 50 mmHg, and anocclusion test is run for at least five minutes.
 2. The method accordingto claim 1, characterized in that the transmural pressure is lowered bythe placing of the cuff, in which pressure is generated, onto theobserved region of the extremity.
 3. The method according to claim 1,characterized in that pressure applied to the tissue of the extremity isincreased gradually, with each step being 5 mmHg and lasting 5-10seconds, at the same time recording the amplitude of theplethysmographic output.
 4. The method according to claim 1,characterized in that in order to lower the transmural pressure in theobserved artery, a mechanical force is applied locally to the tissue ofthe extremity.
 5. The method according to claim 1, characterized in thatin order to lower the transmural pressure in the observed artery, thehydrostatic pressure is lessened by raising the extremity a specifiedheight above the level of the heart.
 6. The method according to claim 1,characterized in that after the value of the transmural pressure hasbeen determined such that the amplitude of the plethysmographic signalis 50% that of the maximal, super-systolic pressure is generated in theocclusion cuff which is placed in close proximity to the observedartery, and the plethysmographic signal is recorded.
 7. The methodaccording to claim 6, characterized in that after at least 5 minutes ofthe occlusion by the cuff, placed in close proximity to the observedartery, pressure in the cuff is lowered to zero, and changes in theplethysmographic signal are recorded simultaneously by the referencechannel and the monitoring channel for at least 3 minutes.
 8. The methodaccording to claims 1 and 7, characterized in that the recordedplethysmographic signal after the occlusion test is analyzed using boththe amplitudal and temporal analyses based on the data collected throughthe reference and monitoring channels.
 9. The method according to claim8, characterized in that during the amplitudal analysis from thereference channel are compared to those collected from the monitoringchannel, the rate of the increase of the amplitude in the monitoringchannel is analyzed, the relationship of the amplitude of the maximumoutput recorded during various transmural pressures is compared to themaximal amplitude of the output recorded after the running of theocclusion test.
 10. The method according to claim 8, characterized inthat during the temporal analysis, plethysmographic waveforms collectedthrough the reference and monitoring channels are compared, the outputis then normalized and the time delay or phase change is determined. 11.Apparatus for noninvasive assessment of endothelial function comprisinga double-channeled sensor unit, capable of detecting pulse wave signalsfrom peripheral arteries, a pneumatic unit capable of creating graduallyincreasing pressure in the blood pressure cuff, and an electronic blockcapable of measuring the pressure generated in the cuff, correspondingto the maximal amplitude of the plethysmographic output, and operatingthe pneumatic unit for generating pressure in the cuff corresponding tothe amplitude of the plethysmographic output, corresponding to anassigned percentage of the maximal amplitude, in addition, the sensorunit is connected to the electronic unit, which is in turn linkedthrough an outlet to the pneumatic unit.
 12. Apparatus according toclaim 11, characterized in that the pneumatic unit is capable ofgenerating pressure in the cuff which is increased gradually byincrements of 5 mmHg with time increments of five to ten seconds. 13.Apparatus according to claim 11, characterized in that each channel ofthe sensor unit comprises an infrared diode and a photoreceptor, bothsituated with the capability to record the light signal passing throughthe observed field.
 14. Apparatus according to claim 11, characterizedin that each of the channels of the sensor unit comprises an infraredlight diode and a photoreceptor, both situated with the capability torecord the diffused light signal reflected off the observed field. 15.Apparatus according to claim 11, characterized in that the sensor unitcomprises impedance metric electrodes, or Hall sensors, or an elastictube filled with electro-conductive material.
 16. Apparatus according toclaim 13, characterized in that the light receptor is connected to afilter with the capability of filtering pulse wave component out of thegeneral signal.
 17. Apparatus according to claim 14, characterized inthat it additionally comprises orthogonally-placed polarized filtersprotecting the photoreceptor from extraneous exposure to light. 18.Apparatus according to claim 11, characterized in that the sensor blockcomprises means for maintaining a designated temperature of the observedregion of the body.
 19. Apparatus according to claim 11, characterizedin that it comprises a liquid-crystal screen for displaying the resultsof endothelial tissue assessment, and/or an interface system connectedto the electronic block for transferring the collected data to acomputer